1. Field of the Invention
The present invention generally relates to a wireless packet communication apparatus and method, and particularly relates to a packet priority control apparatus and a method thereof in a wireless communications system.
2. Description of the Related Art
The W-CDMA method, one of the wireless interfaces of the IMT-2000, enables multimedia access of not only voice but also video images, electronic mail, and the Internet, etc., at a maximum rate of 2 Mbps stationary (at a maximum rate of 384 kbps for moving outdoors at high speed).
A standardization effort is being carried out on detailed technical specifications of the W-CDMA in the 3GPP (the Third Generation Partnership Project), and also a standardization of the HSDPA (the High Speed Downlink Packet Access) for implementing high-speed downlink packet transmission at an even higher rate of 10 Mbps is being carried out.
Moreover, for the purpose of transmitting packet data in a high-speed packet transmission downlink, the cdma 2000 1×EV HDR (High Data Rate) is being studied by the 3GPP2, enabling an implementing of high-speed packet transmission at a maximum rate of information transmission of 2.4 Mbps (refer to Non-Patent Document 1, for example) according to the HDR.
In the high-speed packet transmission as described above, applying of technologies such as Adaptive Modulation and Coding (AMC) based on adaptive wireless-link control (a link adaptation) and Automatic Repeat Request error-control schemes (refer to Non-Patent Document 2, for example) is being studied. The Adaptive Modulation and Coding based on the link adaptation is a method of switching the number of levels in multilevel data modulation, the Spreading Factor (SF), the number of code channels multiplexed, as well as the error-correction code rate depending on the propagation environment of each user, in order to efficiently conduct high-speed data transmission. For the data modulation, increasing the maximum throughput of the system is enabled by switching to a more efficient multi-level modulation, for example, 8PSK, 16QAM, or 64QAM modulation, from QPSK modulation used in the current W-CDMA, as the propagation environment gets better. As an example, ultra-high speed data transmission at 10.8 Mbps is enabled by using a W-CDMA wireless interface at a chip rate of 3.84 Mcps when using 64QAM for the data modulation, assuming SF=16, the number of code channels multiplexed being 10, and the error-correction code rate of ¾.
In order to efficiently implement these transmissions as described above, high-speed packet scheduling is required such that a base station monitors by time slot the state of the propagation path of a mobile station connected to the base station as described above and transmits packets at high speed based on the outcome of the monitoring. In the Non-Patent Document 1 concerning the HDR as described above, a method of maximizing the throughput within a sector by transmitting packets to a mobile station with the best reception characteristics is proposed. However, with this method, unfairness is caused such that packets are not transmitted depending on the mobile station. Therefore, in order to correct for this unfairness, a method is required of taking a ratio of the average of previous transmission rates and an instantaneous transmission rate (the instantaneous transmission rate determined depending on the state of the propagation path at that time), and transmitting packets to a mobile station with a large value of the ratio (refer to Non-Patent Document 3, for example). According to this method, both improving the sector throughput and maintaining fairness among the mobile stations are enabled.
Now, in order to provide full-fledged multimedia services such as a bi-directional video telephone service and a video streaming service, it is necessary to contain the packet transmission delay to within a tolerance demanded by the services for realizing a required transmission rate. However, in the two methods as described above, the probability of not being able to fulfill the quality requirement demanded by the services increases as transmission control taking into account the transmission delay and the required transmission rate is not performed. In general, when scheduling is performed taking into account only the delay tolerance and the required transmission rate, the throughput of the overall system is reduced in comparison with a case wherein the scheduling is performed taking only the reception state into account. To the contrary, a method is proposed such that, for communications not meeting the required quality, by taking into account the reception state of the destination mobile station, the throughput of the overall system is improved while fulfilling the required quality of each communication (refer to Non-Patent Document 4, for example).
Non-Patent Document 1
CDMA/HDR: A Bandwidth-Efficient High-Speed Wireless Data Service for Nomadic Users, Paul Bender, Peter Black, Matthew Grob, Roberto Padovani, Nagabhushana Sindhushayana, and Andrew Viterbi-IEEE Communications Magazine, July 2000
Non-Patent Document 2
Symbol Rate and Modulation Level Controlled Adaptive Modulation/TDMA/TDD for High-Bit-Rate Wireless Data Transmission, T. Ue, S. Sampei, and N. Morinaga: IEEE Transactions on Vehicular Technology, pp. 1134-1147, vol. 47, no. 4, November 1998
Non-Patent Document 3
Data Throughput of CDMA-HDR High Efficiency-High Data Rate Personal Communications Wireless System, A. Jalali, R. Padovani and R. Pankaj, IEEE 51st Vehicular Technology Conference, May 2000
Non-Patent Document 4
Ono et al., “Proposal of ALL IP Mobile Network Architecture (Part 3)—Study of QoS-guaranteed Packet Scheduler in Base Station”, Technical Report of IEICE, MoMaC 2002-3
However, in the related-art method as described in the Non-Patent Document 4, as the reception state of the destination mobile station for communication with a packet-transmission priority is not taken into account, the overall system throughput may end up decreasing when communications not fulfilling the required quality increase due to an increase in traffic, etc. Therefore, there is a problem such that the probability of packet loss increases, especially in a service with a strict requirement for the delay since the quality at one time falls below the required quality level in order for a packet of a given communication to be transmitted with a priority.